This invention relates to paper-making apparatus and more particularly to apparatus for aiding water removal from a moving paper web by application of steam.
Certain paper grades demand uniform sheet properties across the width of the web (in the cross-machine direction). In other grades, such properties are valuable. If this uniformity is to be achieved when using steam to help remove water from a paper slurry or web, accurate control of the heat transfer is essential. Although some systems have used steam to increase water removal from the web and thereby improve the uniformity of the sheet properties, these systems offer only coarse control over the amount of steam applied to each portion of the web. They do not provide the desired degree of uniformity.
One reason for the prior art systems' inability to obtain better uniformity is their failure to recognize the importance of dwell time, i.e., the amount of time in which the steam is in contact with the paper. For most applications, dwell time governs the amount of heat that is transferred from the steam to the paper. The prior art systems do not control dwell time; they only supply or shut off steam to a section of the width of a web. Those sections of the paper that receive no steam do not receive the benefits of improved water removal.
In addition, the systems in the art profile the application of steam to the web only in the cross-machine direction. Thus, these systems can not vary the application of steam along the length of the web, i.e., in the machine direction. This limits the systems' control over water removal so that uniform sheet properties may be difficult to obtain.